Electrolytic refining of metallic aluminum



I Patented July 28 1931 UNITED STATES WILLIAM D. TREADWELL, OF ZURICH,SWITZERLAND,-ASSIGNOR TO ALUMINUM IN- PATENT OFFICE I DUS'IRIEAKTIENGESELLSCHAFT, F NEUEAUSEN, SWITZERLAND, A JOINT-STOCK COMPANY OFSWITZERLAND ELECTROLYTIC REFINING OF METALLIC ALUMINUM No Drawing.Application filed November 30, 1929, Serial No. 410,878, and in GermanyNovember 30, 1928.

This invention relates to the electrolytic refining of metallicaluminum, and consists in a novel process for producing highly puremetallic aluminum from less pure metal.

- The invention is directed particularly to the extraction of refinedaluminum from pig or ingot aluminum, aluminum alloys and the like. Itconstitutes an improvement more particularly in that refining processeswherein the electrodes, used in a solid form, are im- 'mersed in amolten electrolyte, for example containing halogen salts, whichelectrolyte has a melting point lower than that of either electrode andis maintained in a molten con dition below the melting point of eitherelectrode] Such a process has been described in and made the subject ofpatent of Weber and Hauser, No. 1,7 09,759,'granted April 16, 1929, andof, application of Weber and Zeerleder, Serial No. 195,861, filed June1, 1927, which may be referred to for details and features not, hereinfully described.

The general object of the present invention is to render more efficientthe refining process, and to enlarge the rate of output or produc tion.A particular object .is to provide a treatment or step for purifying theelectrodes upon their removal from the electrolytic bath by freeing themfrom the salts ofv the bath which tend to adhere thereto. .A furtherobject is to increase the practical economy of the process by recoveringand rendering available for further use substantially the entirequantity of electrolyte material so ad.- hering to and removed with theelectrodes, and without impairment or change in the chemical compositionthereof. Other objects and advantages of the invention will be explainedin the hereinafter following description of an illustrative embodimentthereof or will be understood to those conversant with the subject. Tothe attainment of such objects and advantages the present inventionconsists in the noval electrolytic refinin process and the novel steps,operations and agents herein described.

The present improved process can best be explained by first pointing outcertain conditions which pertain to the process as already known and theconsideration in vo ved in further perfecting the same. In practicingthe described refining process I have observed that on lifting theelectrodes out of the electrolytic bath they are in an undesirablecondition due to carrying or being covered with substantial quantitiesof the salts constituting the electrolytic bath. The amount of suchelectrolyte material is substantially larger in the case of the anodesfrom which the aluminum has been exhausted than in the case of cathodes.This is due to the fact thatthe electrolyte not only adheres to thesurface of the electrodes, but tends also to penetrate into the narrowcanals and pores left in the anode by the extractionof aluminum, whichlatter may in some cases be replaced by an aproximately equal volume ofsalts ofthe electrolyte.

In the case of the anode, and taking the.

specific gravity of the electrolyte as 1.7 then the exhausted anode willcontain for every kilogram of refined aluminum removed from it 680 gramsof electrolyte material, in'addition to the amount thereof which adheres,to the surface. We have found that the total amount of the electrolytewhich cannot readily-be separated by ;mechanical means from the anodewill amount in general to 700 to 800 grams for every kilogram of refinedaluminum, when the specific gravity of the electrolyte is 1.7.

This substantial loss of electrolyte mate.

rial impaired the economy and efiiciency of the process, and constituteda practical problem in the perfecting of the process. dditionally, thesubsequent working and utilization of the electrodes is rendered muchmore difiicult by reason of the presence of the electrolyte adhering tothem, especially because the readily volatile aluminum chloride compelsspecial precautions to be taken in the melting of the electrodes.Furthermore the strongly hydroscopic character of the electrolyterenders it impracticable to storethe anodes in uncleaned condition afterremoval fromthe bath, since the electrolyte material del iquesces underthe influence of atmospheric moisture and tends to produce crumbling ofthe electrodes.

It is entirely insufficient and impractical x to wash the removedelectrodes with water, for several reasons. Thus, the electrolyte cannot be easily recovered from the aqueous solution owing to thedecomposition of the "aluminum salts. Further, suchan aqueous manner,leaving the electrodes in suitable condition, and conserving the saltsfor repeated use.

' readily recovered by the ill The present improved process is basedupon the determination that the salt mixture employed as an electrolytein the previously described process of refining metallic aluminum isreadily soluble in anhydrous sulfur dioxide. This solvent is furtheradvantageous in that no decomposition of the salts forming theelectrolyte take place, and moreover the entire amount of such salts arequite mere distillin oil of the solvent. This capability of anhyd roussulfur dioxide to dissolve in substantial quantities such a salt mixturewas not to be expected, since the respective salts which enter thecomposition of the electrolyte, if treated separately, are dissolvedonly in very small amounts, insufiicient for practical purposes. Thusfor example, 100 parts of anhydrous sulfur dioxide will dissolve at mostfive parts of the chlorides of aluminum or sodium or potassiumrespectively if brought individually into contact with the solvent.

e same quantity of the solvent will however dissolve a far greaterproportion, in some cases up to 170 parts, of an electrolyte such asthat described in the prior application of Weber and k. Zeerleder,containing these same chlorides of aluminum, sodium and potassium,

with more than one molecule of aluminum chloride present foreachmolecule of the alkali chlorides.

The treatment of the electrodes with the described solvent consisting ofor containing anhydrous sulfur dioxide may be carried out by dipping orimmersion in any sort of extraction apparatus or vessel commonly usedvin the chemical industries, provided that they are adapted to use withthe said solvent. I prefer the use of apparatuses which permit acontinuous extraction of the salts as for instance known apparatusesconstructed in accordance with the Soxhlet principle. However, thetreatment may be otherwise. It is even feasible to bringthe electrodesinto contact with gaseous sulfur'dioxide provided that the latter iscaused to condense upon the dioxide S02,

, specla'li electrodes, since in the gaseous state it of course can notact as a solvent.

The present invention may be summed up specifically as relating to thatprocess of producing refined metallic alum1num comprising removingaluminum electrolytlcally from a solid metallic anode containing 1mpuremetallic aluminum on to a solid cathode through a molten electrolyticbath capable of acting selectively to dissolve aluminum from the anode,which bath contains aluminum halide, and the halides of one or moremetals, such as the alkali metals or the alkali earth metals, thehydroxides of which have alkaline properties, and the improvementcharacterized in that the electrolyte material which is absorbed by oradheres to the'elecof solid electrodes in an electrolyte of moltensalts, it is now with the present invention to perform the step ofclearing or freeing one or both of the electrodes of salts adheringthereto upon their removal from the electrolyte by treatment with asuitable volatile solvent, thus not merely cleaning the electrodes butrendering the electrolyte material available for further use. While wehave disclosed as a suitable volatile solvent anhydrous sulfur variouserably anhydrous, solvents are available, anion which may be mentioned$001 and SO Ci, which however, although effective as solvents, are notso advantageous in a commercial sense on account of their higher costand greater difiiculty in handling.

There has thus been described a process for producing pure metallicaluminum from the less pure metal, together with special treatments orprocedure for the improvement of the process and the product thereof,and a agent or solvent for use therein, embodying the principles andattaining theobjects'of the present invention. Since various matters ofprocess, ingredient and apparatus may be variously modified withoutdeparting om*the principles of the invention, it is not intended tolimit the invention to such matters except so far as set forth in theappended claims.

at is claimed is:

1. The lic aluminum, comprising removing aluminum electrolytically froma solid metallic this solvent being afterwards process of producingrefined metal= 103 other volatile, and prefanode containingimpure'metallic aluminum, on to a solid cathode, through a moltenelectrolytic bath capable of acting selectively to dissolve aluminumfrom the anode, which bath contains aluminum halide, and halides of oneor more metals the hydroxides of which have alkaline properties andcharacterized in that the electrolyte absorbed by or adher- I ing to theelectrodes after their removal from lo the electrolytic'bath isseparated from them and recovered by treating the electrodes withanhydrous sulfur dioxide, the anhydrous sulfur dioxide being afterwardsdistilled off from the resulting solution, leaving the salt mixtureavailable for further use.

2. In the process of electrolytically refining aluminum comprising theemployment of solid electrodes in a molten electrolyte comprising amixture of halide salts, the step of clearing one or both electrodes ofsalts adhering thereto upon removal from the electrolyte by treatmentwith a volatile anhydrous solvent.

3. In the process of electrolytically refining aluminum comprising theemployment of solid electrodes in a molten electrolyte comprising amixture of aluminum halide and the halides of one or more metals thehydroxides of which have alkaline properties, the step of clearing oneor both electrodes of salts adhering thereto upon removal from theelectrolyte by treatment with a volatile solvent containing anhydroussulfur dioxide.

4. In the process of electrolytically refiningaluminum comprising theemployment of solid electrodes in an electrolyte of molten salts, thestep of clearing one or both electrodes of salts adhering thereto uponremoval from the electrolyte by treatment with 40 a solvent consistingof a volatile anhydrous agent. 5. In the process of electrolyticallyrefining aluminum comprising the employment of solid electrodes in anelectrolyte of molten salts, the step of clearing one or both electrodesof salts adhering thereto upon removal from the electrolyte by treatmentwith a solvent consisting of a volatile anhydrous sulfur compound. r Intestimony whereof, this specification has i been duly signed by: lWILLIAM D.- TREADWELL.

cram-"learn or connection. Patent lio. 1,816,304; i Granted 1928, i931;to

VWILLIAM 'D. TREADWELL. it :is hereby certified that thename of theassignee, in the above numbered patent was'erroneously written andprinted as "Aluminum Industrie Aktiengesell'schaft, of Neuhausen,Switzerland, a joint-stock company of Switzerland",

7 whereas said name should have been written and printed as AluminiumIndustrie Aktiengesellschaft, of Neuhausen, Switzerland, a joint-stockcompany. of Switzerland, as shown by the records of assignments in thisoffice; and that the said Letters Patent should be read with thiscorrection therein that the same may conform to the record of the casein the Patent Office.

Signed and sealed this 22nd day of September,.A. D. 1931.

\ M. J; Moore,

(Seal) 7 Acting Commissioner of Patents.

